The invention concerns a roll for a pulp machine, paper machine, or paper finishing machine, said roll comprising a roll mantle of cast iron or steel or any other, equivalent metal, which bears the mechanical loads, as well as a method for the manufacture of the roll and the use of such a roll.
Coated rolls are used in paper machines and paper finishing machines in highly varying applications. As examples of such applications should be mentioned press rolls, suction rolls, soft rolls of calenders and supercalenders, and the equivalent. In different applications and in different processes, different quality requirements are imposed on the roll coating. Usual quality factors of a coating are, among other things, hardness at a certain temperature, resistance to temperature, resistance to compression, chemical strength, smoothness of its face, resistance to mechanical damage, elasticity, surface energy, electrical conductivity, and resistance to ageing.
Thus, more specifically, what is required from paper machine rolls and mainly from roll coatings are surface properties such as hardness/ wear resistance, e.g., in the case of G-rolls and calender rolls, resilient surface, nip and oscillation properties, e.g., in the case of soft-faced press rolls, friction properties, e.g., in the case of paper guide rolls and central rolls, and resistance to corrosion, e.g., in the case of all rolls in the wet end.
The soft coatings on rolls are usually made of organic polymers or mixtures of these polymers which frequently also contain inorganic elements. The soft coatings on rolls are frequently composite constructions, which comprise layers made of different materials.
A problem, e.g., in the case of supercalenders is the poor resistance to heat of coatings made of natural fibres, whereby the high surface temperature of a metal-faced roll, which is advantageous in view of the calendering result, cannot be utilized to the full extent. An increased temperature also restricts an increasing of the compression pressure in calender nips. The roll face is readily damaged in operation. On the other hand, synthetic materials are highly expensive, and limiting factors in the choice of synthetic materials include resistance to heat, hardness, elasticity, and properties of surface energy.
Traditionally attempts have been made to modify the coating properties of polymer-coated rolls by inserting into the coating mixing various additives and fillers. By means of such mixing, attempts have been mostly made to modify/improve properties of the polymer such as cost, wear resistance, resistance to corrosion, thermal conductivity, stiffness, properties of detaching, friction, properties of hysteresis, etc. As one of the problems associated with the mixing it should be mentioned that, when a polymer material is mixed to modify some property, at the same time some other property important for the operation deteriorates. For example, there is a polymer material which operates very well in view of the operation of a press roll, which means that the nip width is good, there are no problems with respect to hysteresis heat, the running quality is good, the adhesion to the metal frame is good, and, moreover, it endures surface pressures and attenuates oscillations, but a residual problem is that the wear resistance is not sufficiently good. In traditional methods, to improve the wear resistance hard, finely divided particles which increase the wear resistance are mixed into the polymer. This mixing increases the wear resistance of the coating, but at the same time it deteriorates properties vital for the press roll, and, as a result, the nip width is reduced, the ability to attenuate oscillations is lowered, the adhesion to the metal frame becomes weaker, the running quality suffers, and in some cases also the resistance to corrosion is reduced. In the worst cases, the final result may be an unusable roll.
Generally speaking, in respect of the prior art regarding the technique of mixed coatings, the Applicant's FI Patent 72,073 is mentioned, wherein a press roll is described whose surface layer is composed of a mixture of a metal powder and an inorganic material. The function of the metal is to act as a binder agent and to increase the toughness of the roll coating, and the function of the inorganic material is to provide a wear-resistant surface of suitable surface energy, because the surface energy of the roll must be within certain limits in order that the detaching of the paper web from the face of the press roll should be controlled. The Applicant's FI Pat. Appl. No. 853544 is of a type similar to that mentioned above. As prior art, reference is also made to the Applicant's FI Pat. Appl. No. 882006, wherein the coating is composed of a powder which consists of carbide-rich powder and of matrix powder.
In the prior art, various multi-layer constructions of rolls and processes for their manufacture are also known. As an example should be mentioned the FI Patent No. 66,667 (Eduard Kusters), wherein, on the roll core, there are a number of rings made of steel in the axial direction, and onto the cylinder core an inner layer and an outer layer have been fixed, which are surrounded from the outside by said rings. A layer construction is also known from the FI Patent No. 55,457 (Beloit Corporation), wherein a combination roll is described, which comprises three concentric roll mantles made as separate casts, a metallurgical bond being provided between said roll mantles. Also, from the FI Pat. Appl. No. 880592 a roll is known which is provided with a multi-layer coating.
One polymer coating and a method for its manufacture are known from the FI Pat. Appl. No. 854748.